1. Try AirDroid under another WiFi network. If it also fails, then it's probably something with your device.
2. Install WiFi File Explorer (https://market.android.com/details?id=com.dooblou.WiFiFileExplorer) and see if it also fails under same network to check if the issue is with your network or your device.
3. Check if any VPN or Firewall setting is blocking the connection between your device and desktop.